1. Field of the Invention
The present invention relates to a method of continuously casting steel slabs with a continuous casting mold, wherein the partially solidified strand composed of a strand shell and a partially liquid core is guided after emerging from the mold during the further solidification between the rollers of a strand guiding unit which has a loose or movable side and a fixed side, and wherein the strand shell forms in the areas between always two rollers spaced apart from each other in the casting direction a deformation in the form of a bulge with an increase of the cross-section.
The present invention also relates to a continuous casting plant suitable for carrying out the method.
2. Description of the Related Art
As a rule, a strand guiding unit suitable for a secure guidance of the cast strand includes a large number of individual strand guiding segments which closely interact in order to guide the strand, wherein the strand guiding segments are provided with guide rollers and/or drive rollers arranged in a loose or movable frame and in a fixed frame, wherein the rollers together form a so-called roller carpet. The rollers may be supported continuously, or intermediate supports may be provided for the rollers in the case of comparatively large strand widths.
Groups of the rollers on the fixed side and the loose side arranged at a distance from the meniscus in the mold have different diameters, while rollers arranged opposite each other to form a pair have the same diameters.
When the strand is guided along a curve between vertical and horizontal sections, the segments are located in the radius ray, starting from the vertical direction underneath the mold up to the horizontal direction.
EP 0 350 431 A2 discloses a continuous casting method for slabs with a continuous casting mold. The strand which is partially solidified over the cross-section thereof is guided between pairs of rollers and is pulled off by means of driven rollers. The individual rollers of pairs of rollers can be adjusted hydraulically relative to the strand so as to deform the strand.
EP 0 535 368 A1 discloses a method and a plant for manufacturing steel strip. In this method, a continuously cast steel strip, which is composed of a solidified strand shell and a liquid core, is reduced in its thickness by deformation by means of rollers and is subsequently rolled.
For preventing undesired thickness deviations and for improving the structure as well as for simplifying the rolling deformation unit, a steel strip strand of 40-80 mm thickness is cast, is deformed by rolling to a thickness of 15-40 mm and 2-15 mm residual liquid in at most three stages and is guided until it is fully solidified.
When carrying out deformation by rolling of a partially solidified steel strand, it has been found that this deformation may lead to strand deformations in the form of bulges of the strand shell between always two adjacent roller contact lines with the strand shell. This produces alternating zones of increased cross-sections and zones of smaller cross-sections, so that the residual melt in the interior of the strand pulsates.
These bulges may lead to critical shell deformations, particularly at the phase border between liquid and solid and, thus, to the formation of internal cracks as a result of the pulsating residual melt movement as a consequence of the periodic bulging of the strand shell and under the influence of the ferrostatic pressure of the liquid steel column which extends up to the meniscus in the mold.
The maximum bulging W.sub.max of the strand shell with liquid core between two adjacent rollers in the casting direction is essentially determined by the following factors:
temperature, T [K] PA1 metallurgical height, h [m] PA1 distance between rollers, L [m] PA1 casting speed, V [m/min] PA1 strand shell, S [mm], wherein PA1 the maximum bulging W.sub.max is determined in the first approximation in accordance with the equation EQU W.sub.max =7,86*10.sup.-12 exp.(0,0046*T) h.sup.2.25 *L.sup.6.1 *V.sup.-0,57 *S.sup.-5,26.
When observing the liquid to be displaced (predominantly a two-phase mixture of crystals and molten steel) during the bulging of the strand shell from roller pair to roller pair, it can be seen that the volume to be displaced builds up a pressure tensor which depends on the ferrostatic pressure, the casting speed, the volume and the travel distance.
In a continuous casting plant, the volume between two pairs of rollers of the roller guiding unit consists of two half volumes which are to be assigned to the fixed side and the loose side of each segment. When this volume of the molten steel is displaced, it exerts a high pressure against the strand shell as it becomes narrower and causes the generation of bulges, while the system strand/strand bulging is superimposed negatively multiplied and leads to uncontrolled movements of the molten steel in the meniscus.